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PANNING
T
REE
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OMMANDS
4-207
Command Usage
• Spanning Tree Protocol
Uses RSTP for the internal state machine, but sends only 802.1D
BPDUs.
- This creates one spanning tree instance for the entire network. If
multiple VLANs are implemented on a network, the path between
specific VLAN members may be inadvertently disabled to prevent
network loops, thus isolating group members. When operating
multiple VLANs, we recommend selecting the MSTP option.
• Rapid Spanning Tree Protocol
RSTP supports connections to either STP or RSTP nodes by
monitoring the incoming protocol messages and dynamically
adjusting the type of protocol messages the RSTP node transmits, as
described below:
- STP Mode – If the switch receives an 802.1D BPDU after a port’s
migration delay timer expires, the switch assumes it is connected to
an 802.1D bridge and starts using only 802.1D BPDUs.
- RSTP Mode – If RSTP is using 802.1D BPDUs on a port and
receives an RSTP BPDU after the migration delay expires, RSTP
restarts the migration delay timer and begins using RSTP BPDUs on
that port.
• Multiple Spanning Tree Protocol
- To allow multiple spanning trees to operate over the network, you
must configure a related set of bridges with the same MSTP
configuration, allowing them to participate in a specific set of
spanning tree instances.
- A spanning tree instance can exist only on bridges that have
compatible VLAN instance assignments.
- Be careful when switching between spanning tree modes. Changing
modes stops all spanning-tree instances for the previous mode and
restarts the system in the new mode, temporarily disrupting user
traffic.